Heretofore there has been a continual need for higher sensitivities, lower noise, higher resolution and higher speeds in infrared-sensitive charge-coupled devices. An overview of these devices, and of CCDs in general, is discussed in "Imaging Devices Using the Charge-Coupled Concept" by David F. Barbe, Charge-Coupled Devices: Technology and Applications, 1977, pp. 130-159.
Narrow bandgap semiconductors, such as mercury-cadmium-telluride (generically denoted as Hg.sub.1-x Cd.sub.x Te, and herein abbreviated as HgCdTe), are extensively employed as the photosensitive semiconductor in infrared detectors. For example, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV, and a 0.1 eV photon has a wavelength of 12 um; whereas Hg.sub.0.73 Cd.sub.0.27 Te has a bandgap of about 0.24 eV, and a 0.24 eV photon has a wavelength of 5 um. These two wavelengths are in two atmospheric windows of great interest for infrared detectors, although other concentrations of HgCdTe useful at other wavelengths are also of interest.
In applications where the image has a velocity relative to the CCD chip, the CCD can be used in the time delay and integration (TDI) mode to enhance the signal-to-noise ratio. In these applications, a CCD is oriented in such a direction and clocked at such a rate that the transfer of charge down the CCD columns is synchronous with the movement of the image along the CCD columns. The noise accumulated during the transfer, e.g. photon shot noise, dark current noise and trapping noise, adds incoherently. The signal-to-noise ratio is improved because the signals add coherently.